The present invention relates to projection display systems for light valves such as liquid crystal display panels, and in particular to the use of color component rotators, such as retardation filters, to provide for improved projection display architectures.
Projection systems for reflective liquid crystal displays (LCDs) are generally characterized by their complexity and large size relative to the systems implemented for transmissive LCDs. FIG. 1A discloses a prior art configuration for a transmissive LCD projector, while FIG. 1B shows a prior art reflective LCD projector for comparison. Dichroic filters DF1 and DF2 separate the red, green, and blue color components. The reflective LCDs require a polarizing beamsplitter (PBS) to be placed in front of each LCD in order to reflect light toward the reflective LCD, and then to transmit the modulated light. These components add complexity to the system and require the use of a larger distribution and recombination optical system to divide the light into the three color channels (i.e. the optical paths traveled by the three color components such as red, green, and blue).
An alternative system for reflective LCDs divides the illumination into the three color channels and recombines the output distributions into a smaller and less complex system. The basic configuration is shown in FIG. 2. However, the illumination input to this system must have a very specific distribution of color components and polarizations in which two of the color components (green and blue) are polarized in one direction, and the other color component (red) is polarized orthogonally to the other two. In order to produce that combination of color components and polarizations, a complicated prefiltering system is needed.
One such system is shown in FIG. 2, and indicates that considerable complexity is added back to the system in order to implement the prefiltering. In the system shown in FIG. 2, only one-half of the light is used, since the unwanted polarization state of each color component is simply discarded. In order to increase brightness of the system, a more complex prefiltering system is required that recycles the polarized light.
Various architectures have been proposed for projection display systems. Ledebuhr, U.S. Pat. No. 4,687,301, and Ledebuhr, U.S. Pat. No. 4,836,649, both describe projection systems for liquid crystal light valve (LCLV). The LCLV is an optically addressed reflective LC modulator and the systems described in these patents show optics to split a light source into separate colors paths and then individually illuminate and project the three LCLV devices. Both of these systems use only one-half of the illumination light since the unwanted polarization state is initially discarded. Ledebuhr, U.S. Pat. No. 4,687,301, uses a complicated color separating system to direct the color components to the LCLVs. Ledebuhr, U.S. Pat. No. 4,836,649 uses simpler but more numerous elements resulting in a large projection system.
Doany, et al. U.S. Pat. No. 5,621,486, and Dove, U.S. Pat. No. 5,658,060, both describe architectures that use Philips type prisms to control the three separate color channels. Doany, et al. U.S. Pat. No. 5,621,486, uses a single PBS prism to control the light into and out of all three LCIs) and a Philips prism to both split up and recombine the color channels. This arrangement appears simple, but the control of color in a Philips prism for p-polarization on the input and s-polarization on the output is extremely difficult, and no successful implementation of this type of system exists. Dove, U.S. Pat. No. 5,658,060, places a P13S prism in front of each LCD and uses the Philips prism only to recombine the color channels. This requires a second optical arrangement to split up the color distributions and leads to a larger, more complicated system overall.
Ooi et al., U.S. Pat. No. 5,648,860, uses an offset illumination and projection scheme. The system does not use a PBS prism, but instead relies on the offset to separate the input and output light distributions. The color splitting and recombination is accomplished by tilted dichroics that perform essentially the same as the dichroics in a Philips prism, with the same polarization related problems.
Hattori et al., U.S. Pat. No. 5,798,819, and Ueda, U.S. Pat. No. 5,918,961, both describe minor variations of the typical reflective LCD projector of FIG. 1B. These systems use a crossed dichroic prism to recombine light from the three LCDs and a separate crossed dichroic arrangement to perform the color splitting from the illumination system.
Sharp, U.S. Pat. No. 5,751,384, describes techniques for making waveband-specific retardation filters. This patent also describes a single panel LCD projector using the retardation filters in an active color shutter to gate the three colors onto the LCD for color field sequential projection.
Nevertheless, there remains a need for a bright projection display system, preferably for reflective LCD panels that utilize a small architecture. What is therefore desired is a projection display that is as small as or smaller than conventional projection displays, is capable of utilizing reflective LCD panels, uses readily available optical elements that perform well, uses conventional polarization converters, and provides good contrast without sacrificing brightness.